Return cdr of coprocessor form in case of error.

[porcuquine]

        // FIXME: technically, the error is defined to be rest -- which is the cdr of input_expr.
        //let new_expr = pick_ptr!(cs, &arity_is_correct, &quoted_expr, &rest)?;
        let new_expr = pick_ptr!(cs, &arity_is_correct, &quoted_expr, &input_expr)?;

https://github.com/lurk-lab/lurk-rs/blob/ee2f0f4d8068024acf5ab453c6b0c458bbd52175/src/coprocessor/mod.rs#L122C1-L125C1

          This one may deserve its issue.

Originally posted by @huitseeker in #677 (comment)